IGBTs are employed as power semiconductor elements that are used in high voltage inverters of hybrid electric vehicles (HEVs) and electric vehicles (EVs). With IGBTs, there is a tendency for the short circuiting toleration to be low as compared to MOSFETs, so that it is necessary to give consideration to design of short circuiting and over-current protection circuitry.
In the design of an inverter, in order to ensure security, it is possible to use IGBTs whose short circuiting toleration is high, but doing so has multiple demerits, such as increase of the cost, increase of steady loss, and so on. Moreover there is also a method of keeping down the short-circuiting current of an IGBT by clamping the gate voltage with a Zener diode when short circuiting occurs with the objective of reducing the energy during short circuiting, but there is a danger of the gate voltage oscillating, depending upon the type of the IGBT.
In Patent Document #1, it is disclosed to suppress over-current by clamping the gate voltage of an IGBT using a Zener diode when over-current is detected. However, with this method, there is a danger that the gate voltage may oscillate due to repetition of Zener clamping operation and Zener clamping cancellation operation. This phenomenon may become a cause for increase in voltage surging, and this is undesirable.